A combustion engine, especially of the piston type, may use a fuel injector for injecting fuel into a combustion chamber. The fuel injector comprises a fuel valve and an electric actuator for the valve. The valve comprises a valve seat and a valve needle for controlling the flow of the fuel. The electric actuator comprises a solenoid and an armature for lifting the valve needle from the valve seat while the solenoid is electrically energized.
When the solenoid is no longer energized, the valve needle of the fuel valve is returned to the valve seat by means of an elastic element and a flow of fuel is stopped. However, there may be an unintended flow of fuel through the injector after the valve needle has impacted on the valve seat. The valve needle of the injector may bounce back from the valve seat and reopen the fuel valve immediately after its closing, which is commonly referred to as “bounce”.
In particular in the case of injectors having the armature axially displaceable with respect to the valve needle, also a reopening of the injector up to 2 milliseconds after the impact may take place, which phenomenon is called “post injection”. It may be caused by an interaction between the valve needle and the armature when the armature returns to a rest position in contact with the valve needle after decoupling from the valve needle when the latter impacts on the valve seat.
A prevention of uncontrolled and unwanted fluid flow through the injector has thus far been addressed with a flow-restrictive device upstream the valve. The device is commonly called anti bounce disc. It serves as a dynamic brake that chokes the main fuel channel, thus reducing the speed of movement of the valve needle when it approaches the valve seat. The anti-bounce disc has the disadvantage that it is within the hydraulic flow path of the injected fuel and causes a hydraulic resistance to the fuel. With the anti-bounce disk, the bounce effect is smaller but the injector needs more time to close and the maximum operative hydraulic pressure is significantly reduced.
There are also other approaches for bounce avoiding, but they are generally not effective for reduction or elimination of post injection.